Fix/anywidget refresh value

[Shamik-07]

📝 Summary

The Counter anywidget was always defaulting to a default value of 5 upon reloading the notebook as the state wasn't being reattached to the FE. This fixes the issue by sending an echo_update to BE and upon reload reconnects to get the correct state. FE calls reemitState ensure that all listeners receive the state even if one was rendered before the other.
Closes #9420

Screen.Recording.2026-05-04.at.17.29.29.mov

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✅ Merge Checklist

• I have read the contributor guidelines.
• Documentation has been updated where applicable, including docstrings for API changes.
• Tests have been added for the changes made.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	May 7, 2026 9:22pm

[cubic-dev-ai]

1 issue found across 6 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="marimo/_plugins/ui/_impl/comm.py">

<violation number="1" location="marimo/_plugins/ui/_impl/comm.py:123">
P3: Update `_create_model_message` docstring: it still says `echo_update` is skipped, but this branch now emits a `ModelUpdate`.</violation>
</file>

Architecture diagram

sequenceDiagram
    participant UI as Widget UI (FE)
    participant Plugin as AnyWidgetPlugin
    participant Model as Frontend Model
    participant Comm as Marimo Comm (BE)
    participant Kernel as Python Kernel

    Note over UI, Kernel: Phase 1: State Capture (During Interaction)

    UI->>Model: model.set("count", 8)
    Model->>Comm: sendUpdate({ method: "echo_update", state: {count: 8} })
    
    rect rgb(240, 240, 240)
        Note over Comm: CHANGED: Handle "echo_update"
        Comm->>Comm: Create ModelUpdate message
        Note right of Comm: Prevents dropping state<br/>on acknowledgement path
    end
    
    Comm-->>Kernel: Broadcast state to all clients

    Note over UI, Kernel: Phase 2: State Hydration (Upon Notebook Reload)

    Kernel->>Comm: Replay stored state
    Comm->>Model: updateAndEmitDiffs({ count: 8 })
    
    Plugin->>Plugin: bind(widgetDef, model)
    Plugin->>UI: render(element, model)
    
    alt Widget Initialization
        UI->>Model: model.on("change:count", listener)
        Note left of UI: Widget defaults to "5"<br/>(Internal initial state)
    end

    rect rgb(240, 240, 240)
        Note over Plugin, Model: NEW: Re-hydration Flow
        Plugin->>Model: getMarimoInternal(model).reemitState()
        
        loop For each key in state
            Model->>UI: emit("change:count", 8)
        end
        
        Model->>UI: emit("change")
    end

    UI->>UI: listener updates UI to "8"

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[cubic-dev-ai]

P3: Update _create_model_message docstring: it still says echo_update is skipped, but this branch now emits a ModelUpdate.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At marimo/_plugins/ui/_impl/comm.py, line 123:

<comment>Update `_create_model_message` docstring: it still says `echo_update` is skipped, but this branch now emits a `ModelUpdate`.</comment>

<file context>
@@ -120,8 +120,14 @@ def _create_model_message(
     elif method == "echo_update":
-        # echo_update is for multi-client sync acknowledgment, skip it
-        return None
+        # Preserve frontend-driven trait changes for reconnect replay.
+        # anywidget/ipywidgets can emit echo_update as the synchronisation
+        # acknowledgement path; dropping it causes stale replay state.
</file context>

[Copilot]

Pull request overview

Fixes anywidget model state hydration on browser refresh/reconnect by ensuring frontend-originated state changes are preserved for session replay and by re-emitting the current model state after a widget view attaches listeners.

Changes:

• Backend: treat echo_update messages as ModelUpdate so they are included in replayable model state.
• Frontend: add an internal reemitState() API to re-dispatch current model values as change:* events after render().
• Tests: add backend and frontend test coverage for echo_update replay contribution and late listener hydration.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
marimo/_plugins/ui/_impl/comm.py	Converts echo_update into ModelUpdate to preserve state for reconnect replay.
tests/_plugins/ui/_impl/test_comm.py	Adds coverage ensuring echo_update is broadcast as a state-bearing update.
frontend/src/plugins/impl/anywidget/model.ts	Adds internal reemitState() that emits change:* events for current state.
frontend/src/plugins/impl/anywidget/AnyWidgetPlugin.tsx	Calls reemitState() after render() so late listeners observe hydrated state.
frontend/src/plugins/impl/anywidget/__tests__/model.test.ts	Tests reemitState() emits field and aggregate change events.
frontend/src/plugins/impl/anywidget/__tests__/AnyWidgetPlugin.test.tsx	Adds regression test for hydration when listener attaches after initial state is set.